Synthetic investigation of competing magnetic interactions in 2D metal-chloranilate radical frameworks

Abstract

The discovery of emergent materials lies at the intersection of chemistry and condensed matter physics. Synthetic chemistry offers a pathway to create materials with the desired physical and electronic structures that support fundamentally new properties. Metal-organic frameworks are a promising platform for bottom-up chemical design of new materials, owing to their inherent chemical predictability and tunability relative to traditional solid-state materials. Herein, we describe the synthesis and magnetic characterization of a new 2,5-dihydroxy-1,4-benzoquinone based material, (NMe₂H₂)_3.5Ga₂(C₆O₄Cl₂)₃ (1), which features radical-based electronic spins on the sites of a kagomé lattice, a geometric lattice known to engender exotic electronic properties. Vibrational and electronic spectroscopies, in combination with magnetic susceptibility measurements, revealed 1 exhibits mixed valency between the radical-bearing trianionic and diamagnetic tetraanionic oxidation states of the ligand. This unpaired electron density on the ligand forms a partially occupied kagomé lattice where approximately 85% of the lattice sites are occupied with an S = ½ spin. We found that gallium mediates ferromagnetic coupling between ligand spins, creating a ferromagnetic kagomé lattice. By modulation of the interlayer spacing via post-synthetic cation metathesis of 1 to (NMe₄)_3.5Ga₂(C₆O₄Cl₂)₃ (2) and (NEt₄)₂(NMe₄)_1.5Ga₂(C₆O₄Cl₂)₃ (3), we determined the nature of the magnetic coupling between neighboring planes is antiferromagnetic. Additionally, we determined the role of the metal in mediating this magnetic coupling by comparison of 2 with the In³⁺ analogue, (NMe₄)_3.5In₂(C₆O₄Cl₂)₃ (4), and we found that Ga³⁺ supports stronger superexchange coupling between ligand-based spins than In³⁺. The combination of intraplanar ferromagnetic coupling and interplanar antiferromagnetic coupling exchange interactions suggests these are promising materials to host topological phenomena.

Fig. 1. (a) Scheme of accessible oxidation states of the redox active chloranilate organic ligand. (b) Crystallographic structure of 1 determined from Rietveld refinement. Grey, red, green, and purple spheres represent carbon, oxygen, chlorine, and gallium atoms respectively. Solvent molecules and cations are omitted for clarity. The kagomé lattice is shown in teal.

Fig. 2. (a) Raman spectrum collected for a solid sample of 1 from 400 to 1900 cm⁻¹ following excitation at 473 nm. The plot to the right highlights the main peak at 1453 cm⁻¹ which has a shoulder at 1440 cm⁻¹. (b) Diffuse reflectance spectrum collected for a solid sample of 1 under a N₂ atmosphere, with arrows highlighting relevant transitions. (c) DC magnetic susceptibility for 1 at 0.1 T. Inset shows magnetization curves from 0 to 7 T at temperatures of 1.8, 5, and 10 K. Lines are guides to the eye.

Fig. 3. (a) PXRD of 1, 2, and 3 collected with Cu Kα₁ radiation showing the expansion of the interlayer spacing with intercalation of tetraalkylammonium cations. The arrows point to the (002) peak of each pattern that corresponds to the interlayer spacing and is modulated by the size of the cation. (b) DC magnetic susceptibility of 1, 2, and 3, displaying the dependence of the low temperature magnetic behaviour on the interlayer spacing. The inset highlights the dependence of χ_MT_max on 1/R³, where R is the interlayer spacing. The data are fit to a linear relationship with an R² of 0.981, demonstrating the strong correlation between the maximum magnetic moment and the interlayer spacing.